The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for detecting the presence of a substrate in a carrier head of a chemical mechanical polishing system.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. Therefore, the substrate surface is periodically planarized surface to provide a substantially planar layer surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted to a carrier or polishing head. The exposed surface of the substrate is then placed against a rotating polishing pad. The carrier provides a controllable load, i.e., pressure, on the substrate to press it against the polishing pad. In addition, the carrier may rotate to affect the relative velocity distribution over the surface of the substrate. A polishing slurry, including an abrasive and at least one chemically-reactive agent, may be distributed over the polishing pad to provide an abrasive chemical solution at the interface between the pad and substrate.
Typically, the carrier head is used to remove the substrate from the polishing pad after the polishing process has been completed. The substrate is vacuum-chucked to the underside of the carrier head. When the carrier head is retracted, the substrate is lifted off the polishing pad.
One problem that has been encountered in CMP is that the substrate may not be lifted by the carrier head. For example, if the surface tension binding the substrate to the polishing pad is greater than the force binding the substrate on the carrier head, then the substrate will remain on the polishing pad when the carrier head retracts. Also, if a defective substrate fractures during polishing, then the carrier head may be unable to remove the fractured substrate from the polishing pad.
A related problem is that the attachment of the substrate to the carrier head may fail, and the substrate may detach from the carrier head. This may occur if, for example, the substrate was attached to the carrier head by surface tension alone, rather than in combination with vacuum-chucking.
As such, an operator may not know that the carrier head no longer carries the substrate. The CMP apparatus will continue to operate even though the substrate is no longer present in the carrier head. This may decrease throughput. In addition, a loose substrate, i.e., one not attached to a carrier head, may be knocked about by the moving components of the CMP apparatus, potentially damaging the substrate or the polishing pad, or leaving debris which may damage other substrates.
Another problem encountered in CMP is the difficulty of determining whether the substrate is present in the carrier head. Because the substrate is located beneath the carrier head, it is difficult to determine by visual inspection whether the substrate is present in and properly attached to the carrier head. In addition, optical detection techniques are impeded by the presence of slurry.
A conventional carrier head may include a rigid base having a bottom surface which serves as a substrate receiving surface. Multiple channels extend through the base to the substrate receiving surface. A pump or vacuum source can apply a vacuum to the channels. When air is pumped out of the channels, the substrate will be vacuum-chucked to the bottom surface of the carrier head. A pressure sensor may be connected to a pressure line between the vacuum source and the channels in the carrier head. If the substrate was not successfully vacuum-chucked to the underside of the carrier head, then the channels will be open and air or other fluid will leak into the channels. On the other hand, if the substrate was successfully vacuum-chucked to the underside of the carrier head, then the channels will be sealed and air will not leak into the channels. Consequently, the pressure sensor will measure a higher vacuum or lower pressure when the substrate is successfully vacuum-chucked to the underside of the carrier head as compared to when the substrate is not properly attached to the carrier head.
Unfortunately, there are several problems with this method of detecting the presence of a substrate in the carrier head. Corrosive slurry may be suctioned into the channels and contaminate the carrier head. In addition, the threshold pressure for determining whether the substrate has been lifted from the polishing pad must be determined experimentally.
Accordingly, it would be useful to provide a CMP system capable of reliably sensing the presence of a substrate in a carrier head. It would also be useful if such a system could operate without exposing the interior of the carrier head to contamination by a slurry.
In one aspect, the present invention is directed to a carrier head for a chemical mechanical polishing system. The carrier head includes a base and a flexible member connected to the base to define a chamber. A lower surface of the flexible member provides a substrate receiving surface. There is an aperture in the flexible member between the substrate receiving surface and the chamber.
Implementation of the invention may include the following. The aperture may be configured such that if a substrate is attached to the substrate receiving surface, the substrate blocks the aperture. If fluid is forced into or evacuated from the chamber and a substrate is attached to the substrate receiving surface, a pressure in the chamber may reach a first pressure which is different than a second pressure that would result if the substrate were not attached to the substrate receiving surface. The carrier head may be part of an assembly including a vacuum source connected to the chamber, a sensor to measure a pressure in the chamber and generate an output signal representative thereof, and a processor configured to indicate whether the substrate is attached to the substrate receiving surface in response to the output signal. The processor may be configured to indicate that the substrate is attached to the substrate receiving surface if the pressure in the chamber is greater than a threshold pressure.
In another aspect, the carrier head includes a base, a flexible member connected to the base to define a chamber, a first passage in the base connecting the chamber to the ambient atmosphere and a second passage in the base connecting the chamber to a passage in a drive shaft. A lower surface of the flexible member provides a substrate receiving surface.
Implementations of the invention may include the following. The second passage may be positioned such that, if a fluid is evacuated from the chamber and a substrate is not attached to the substrate receiving surface, the flexible member deflects inwardly to block the second passage so that a pressure in the second passage drops to a first pressure which is less than a second pressure that would result if the substrate were attached to the substrate receiving surface. The carrier head may include a check valve in the first passage to prevent fluid from exiting the chamber through the first passage. The carrier head may is include a mechanically actuatable valve across the first passage, the valve configured such that if a fluid is evacuated from the chamber and a substrate is not attached to the substrate receiving surface, the flexible member deflects inwardly to actuate the valve.
In another aspect, the carrier head includes a base, a first flexible member connected to the base to define a first chamber, a second chamber in the base, and a valve across a passage between the first chamber and the second chamber. A lower surface of the first flexible member provides a substrate receiving surface.
Implementations of the invention include the following. The valve may be configured such that if fluid is evacuated from the first chamber and a substrate is not attached to the substrate receiving surface, the flexible member deflects to actuate the valve so that a pressure in the second chamber reaches a first pressure which is different from, e.g., less than, a second pressure that would result if the substrate were attached to the substrate receiving surface. A second flexible member may define the second chamber. The second flexible member may be positioned above the first flexible member, and an upward motion of the first flexible member may exert a force on the second flexible member. A pressure source may be connected to the second chamber to pressurize the second chamber. A pressure sensor may measure the pressure in the second chamber at a first time and a second time and generate output signals representative thereof, and a processor may be configured to indicate whether the substrate is attached to the carrier head in response to the output signals. A second valve may isolate the pressure source from the second chamber.
In another aspect, the invention is directed to a carrier head including a base, a first flexible member connected to the base to define a first chamber, a second flexible member connected to the base to define a second chamber, and a passage in the base connecting the chamber to a passage in a drive shaft. The first flexible member exerts a force on the second flexible member. The passage in the base is positioned such that if a fluid is evacuated from the chamber and a substrate is not attached to the substrate receiving surface, the flexible member deflects inwardly to block the second passage so that a first force on the second flexible member is different than a second force that would result if the substrate were attached to the substrate receiving surface.
Advantages of the invention include the following. The CMP apparatus includes a sensor to detect whether the substrate is present or properly attached to the carrier head. The interior of the carrier head is not exposed to slurry. The sensor is able to detect whether a substrate is held on the carrier head by surface tension rather than by vacuum.
Other advantages and features of the invention become apparent from the following description, including the drawings and claims.